6 BOTANICAL GAZETTE [JULY 
wheat seedlings is based has already been described by Livincston® 
and in former bulletins of this bureau.? The method consists in 
having a perforated disk, supported by ordinary corks in such a way 
that it will just float upon the surface of a pan of water. In the earlier 
work a wire gauze, which had been coated with paraffin so as to make 
virtually a plate of paraffin reinforced by wire, was used, holes being 
made in this plate by means of a hot wire. Perforated cork sheets, 
preferably paraffined, have also been used. In this case sheet cork 
_ about one-eighth of an inch in thickness is immersed in melted paraffin, 
and after removal from the paraffin this is allowed to harden in the air — 
spaces of the cork. Holes are then made by means of a small cork 
borer so as to give a perforated plate. Both of these plates are open 
to objections. In the paraffined plate, when it is used continually, 
there is a tendency for the paraffin to split off from the wire, which is 
thus exposed to the action of the water and the roots; moreover, this 
plate is not easily repaired. The cork plate, on the other hand; shows 
considerable tendency to warp, is rather fragile, and is not easily 
kept sterile. 
Instead of these plates there were used in these present experiments 
perforated hard-rubber sheets, thus overcoming the above objections 
to a considerable degree. These are prepared by. cutting a circular 
disk of 305 mm. from vulcanized sheet rubber 3.2 mm. in thickness. 
By clamping several of these tightly together, preferably between 
layers of wood, small holes are drilled through the mass approximately 
4.8mm. in diameter and 5.0mm. apart. Disks of this material 
float level upon the water when supported by corks, are not so sub- 
ject to warping, and are readily cleaned and kept sterile. The corks 
are fastened to the under surface of the disk in four or five places on 
the circumference and in the center. For this purpose either rubber 
or wooden pegs or wire may be used; if the latter, it should be made 
either of iron or aluminum, never of brass or copper: The size of the 
corks is so gauged by trial that the disk is just supported on the surface 
6 Livincston, B. E., A simple method for experimenting with water cultures. 
Plant World 9:13. 1906. 
7 SCHREINER, O., and REED, H. S., Some factors influencing soil fertility. Bulle- 
tin 40, Bureau of Soils, U.S. Dept. Agric. 1907. : 
